Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. Such recording materials become photothermographic upon incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
GB-A 1,542,327 discloses a thermally developable light-sensitive sheet material, comprising a support and (a) a silver salt of an organic acid, (b) a catalyst in an amount capable of catalyzing the reaction in exposed areas of the material of components (a) and (c) after imagewise exposure and heating of the material, (c) a reducing agent for the salt (a), and (d) sulphur in an amount to reduce the colour change after processing and to reduce thermal fogging, the components (a) to (d) being contained, separately or together, in one or more layers coated on the support or being all present in the support or one or more of the components being present in the support and the remainder being in one or more layers coated thereon. The description of GB 1,542,327 includes an exhaustive list of silver salts of an organic acid including silver palmitate. Furthermore, D. S. Avose, V. V. Tsvetkov and V. D. Yagodovskii in Sci. Appl. Photo. volume 35, pages 587-594 published in 1994 by Gordon and Breach Science Publishers S. A. describe the photothermographic materials based on silver bromide and silver palmitate.
U.S. Pat. No. 4,273,723 discloses a process for preparing a silver salt of a fatty acid with 12 to 24 carbon atoms consisting essentially of reacting an alkali metal salt of the fatty acid with a water-soluble silver salt, and wherein the reaction is effected in a reaction system consisting essentially of (I) the alkali metal salt of the fatty acid, (II) the water-soluble silver salt, (III) at least one water-soluble or partially water-soluble C.sub.3 -C.sub.8 alcohol and (IV) water, the volume ratio of the component (III) to the component (IV) being 1/5 to 5/1.
GB-A 1,378,734 discloses a process of producing a silver salt of an organic carboxylic acid conducted in the presence of a soluble mercury compound and/or a soluble lead compound. EP-A 754 969 discloses a process for producing a suspension of particles containing a substantially light-insensitive silver salt of an organic carboxylic acid, comprising simultaneous metered addition of an aqueous solution or suspension of an organic carboxylic acid or its salt; and an aqueous solution of a silver salt to an aqueous liquid, wherein the metered addition of the aqueous solution or suspension of the organic carboxylic acid or its salt; and/or the aqueous solution of the silver salt is regulated by the concentration of silver ions or the concentration of anions of the silver salt in the aqueous liquid. Research Disclosure number 17029, published in June 1978,gives a survey of different methods of preparing organic heavy metal salts in section II.
The association of silver palmitate with mercury or lead ions, particularly mercury ions, according to the teaching of GB 1,378,734, is environmentally undesirable and infringes governmental regulations.
Recording materials with prior art silver palmitate exhibit poor shelf-life, particularly as regards increase in D.sub.max.